well, use that website and write down their description of the clay powder. Plantex CSM will be reasonbly easy to find online, but locally...I'm not so sure.
A pound of CSM would run about $12. A pound of that clay powder is probably a $1.
And that'd probably make 300-500 root tabs.
I'd search online to find the types of stores that sell the powder, then try to find a similar local store. But you may have little luck. In my small city of 250,000, I can't get stuff locally.
The garden centers/nursuries here didn't know what potassium nitrate was, so of course they didn't carry it. plantex CSM is even more 'specialized' I think.